Different supply voltages are required by portable electronic devices such as netbooks, personal-digital-assistance (PDA) phones and portable media player (PMP). DC-DC switching converters are commonly used as power supplies for these devices because of their high efficiency. Single-inductor-multiple-output (SIMO) DC-DC converters are an important component to provide multiple supply voltages for these applications. With a reduced number of inductors, SIMO DC-DC converters provide a solution with relatively small size and reduced cost.
To achieve a high performance SIMO DC-DC converter, many design parameters should be taken into account such as load transient response, cross-regulation, and output ripple. U.S. Patent Pub. No. 2008/0231115, U.S. Pat. No. 7,432,614 and U.S. Patent Pub. No. 2011/0043181 describe some conventional control techniques, such as Ordered-Power-Distributive-Control and Time-Multiplexing or Sequential-Control for SIMO DC-DC converters. For these converters, the required energy is determined by error amplifiers. The load transient response is thus limited by the compensator.
U.S. Patent Pub. No. 2008/0231115 describes a control method where the inductor accumulates energy once and transfers the energy to multiple outputs one by one based on the predefined priority in one cycle where the last output stage uses peak-current-mode control. This last stage determines the response and the cross-regulation of the converter. For this type of control, if the first output has a heavy load, the rest of the outputs might not be able to get energy for many cycles due to the defined energy transfer priority, which induces voltage drops at all the other outputs.
For converters utilizing control methods similar to those described in U.S. Pat. No. 7,432,614 and U.S. Patent Pub. No. 2011/0043181, the system should operate in Discontinuous-Conduction-Mode (DCM) to minimize cross-regulation, which either limits the power capability of the system or leads to a relative large ripple current and voltage at heavy load. Utilizing Pseudo-Continuous-Conduction-Mode (PCCM) or Continuous-Conduction-Mode (CCM) operation for this type of control will reduce the effectiveness of the cross-regulation suppression, increase the power loss and lead to a complicated design.